This invention relates to a plastic encapsulated semiconductor device and, in particular, to a plastic encapsulated semiconductor device which allows to mount the largest semiconductor element (for example, a semiconductor chip) possible within limited outside dimensions and which is suitable for preventing generation of resin cracks.
In conventional plastic encapsulated semiconductor devices, a structure has been employed in which the semiconductor element (or chip) is fastened to an element (or chip) mounting section called a tab and a plurality of leads are provided around this tab and terminals provided on the semiconductor element are electrically connected to these leads through metal wires and the components thus connected together are sealingly molded by resin.
Nowadays, the size of the semiconductor element (or chip) is gradually increasing as its degree of integration becomes higher, whereas the outside dimensions of the semiconductor device, in which this element is to be mounted, cannot be increased at will due to the requirements for high-density mounting or the tendency is rather toward a decrease in its size. Thus, if the size of the semiconductor element is increased, with the outside dimensions of the semiconductor device remaining unchanged, the conventional semiconductor-device structure in which the semiconductor element is mounted on a tab develops a problem in which the length of a portion for connecting the leads with therein becomes insufficient, so that the leads cannot be secured in position with sufficient fixing strength.
As a means for avoiding this problem, a structure as shown in FIG. 10 is disclosed in Japanese Patent Unexamined Publication No. 61-241959. In this known structure, two insulating films 2 are glued to the circuit-forming surface of a semiconductor element 1, and two common leads 3a for electrical connection are respectively attached to the two insulating films 2 at positions in the vicinity of the inner ends of the films so as to be opposed to each other. Further, signal leads 3b are attached to the insulating films 2 at positions spaced away from the common leads 3a, with the common leads 3a and the signal leads 3b being connected to the semiconductor element 1 through metal wires 4. This type of structure is generally referred to as the "lead-on-chip structure". In another known structure using no tab, which has been proposed, likewise, as a means of avoiding the above problem, a surface of the semiconductor element 1 which is reverse to the circuit forming surface, is attached to the leads 3. The structure is the reverse of the lead-on-chip structure and is referred to as the "chip-on-lead structure". An example of the chip-on-lead structure is disclosed in Japanese Patent Unexamined Publication No. 61-258458.
First, a problem in the prior art will be discussed with reference to the lead-on-chip structure.
Generally, a high molecule material such as polyimide is used as a base material of the insulating film. The base material has a poor adhesive to the encapsulating plastic. It is necessary for the circuit forming surface of the semiconductor element to be electrically connected to the leads by the metal wires. In view of this, the provision of the insulating film is effected only partially, that is, the film is glued only to those sections of the element where they are needed.
In a plastic encapsulated semiconductor device, the respective coefficients of linear expansion of the semiconductor element, leads, insulating film, and encapsulating resin which constitute the device, are usually different from each other, so that any temperature change in the device may cause thermal stress therein. This difference in coefficient of linear expansion is particularly large between the insulating films and the encapsulating resin. Thus, combined with the poor adhesiveness as mentioned above, the interface between these two components is particularly subject to separation, which is easily caused by any thermal stress in the device. When an interface separation occurs between the insulating film and the encapsulating resin, a high stress is generated in the upper end sections of the insulating film portions, causing resin cracks in those end sections.
FIG. 9 schematically shows a resin crack generation mechanism. In this example, crack generation is caused as a result of cooling after plastic encapsulation of the semiconductor device and decrease in temperature at the time of temperature cycle test. As stated above, the coefficients of linear expansion of the device components are different. In the device section above the semiconductor element 1, this difference in the coefficients of linear expansion causes the encapsulating resin 5, which would normally contract as a result of cooling, to be pulled by the semiconductor element 1, with the result that the outer edge surfaces 2c of the insulating film portions are separated from the resin 5, as indicated at 7. As a result of this separation, a stress concentration occurs in the upper end sections of the insulating film portions, thereby causing resin cracks 8b. This also applies to the sections where leads are positioned near the edge surfaces of the insulating film portions. That is, when, in FIG. 9, an interface separation as indicated at 7 occurs between the resin 5 and the inner edge surface 2b of the insulating film portions, this separation also develops in the interfaces between the edge surfaces of the common leads 3a and the encapsulating resin 5, with the result that a concentration of stress occurs in the upper end sections of the common leads 3a, thereby causing resin cracks 8a there.
Generally, the generation of resin cracks not only impairs the sealing property and appearance of the semiconductor device, but, in the case where cracks 8a are generated at the upper end sections of the common leads 3a, causes disconnection of the metal wires 4, which electrically connect the common leads 3a or the signal leads 3b to the semiconductor element 1.
The above problem, which has been described with reference to the lead-on-chip structure, also applies to other types of plastic encapsulated semiconductor devices using insulating films.